Leeneia,

You can edit or create a MIDI with any timing you want but those timings are played back in real time.

MIDI has no concept of half notes, quarter notes, minims, crotchets, or many things notational it doesn't need them. It uses a series of events, eg. what key is pressed (pressed could be how you entered it in your notation program) and when these events are to occur in the piece.

Sometimes a program is able to convert these events into notes on a musical score, other times it isn't. That a program fails need not be a poor reflection on the program nor an indication that the MIDI is incorrect in any way.